This invention relates to a radio frequency (“RF”) object location method and apparatus. More specifically, this invention relates to system architecture, as well as an improved synchronization method and apparatus for locating an object in a multipath, reverberation, or other noisy environment.
RF location systems may be used to keep track of objects such as inventory, materiel, equipment, personnel, and other items. In such systems, objects to be located typically utilize individual transmitters or transponders, i.e., RF tags. To locate the object, various techniques have previously been used to process received signals.
In one prior systems, RF sensors (also referred to as “monitoring stations”) were positioned at known coordinates within and/or about an area to be monitored. RF emissions from tags associated with the object were received and processed by these sensors. Signal processing schemes included measuring relative signal strength, angle of arrival (AOA), or time difference of arrival (TDOA or DTOA). Typically, systems based upon TDOA determined differences in the arrival time of the signal from the tag at one monitoring station relative to other monitoring stations. Measurement of time difference was often accomplished using a digital counter whose count was latched in response to receipt of an incoming RF signal. Systems based upon such TDOA measurements were sometimes referred to as a “multilateration” or “geolocation” system, which refers to the process of locating a signal source by solving for the mathematical intersection of multiple hyperbolae, which was determined by the difference of arrival times between signals received at multiple sensors.
In another class of prior systems as described, for example, in U.S. Pat. No. 4,740,792 and commonly-owned, incorporated U.S. Pat. No. 6,054,950, untethered monitoring stations relayed received signals via wireless links to a central measurement unit. Although well-suited for monitoring object location in large outdoor areas, or in applications where wiring was not feasible or too expensive to install, this approach required a transmitter and receiver at each station.
In another class of prior systems (cf. U.S. Pat. Nos. 3,714,573; 5,216,429; 5,920,287; and 6,121,926), tethered monitoring stations relayed radio frequency signals via cables to a central measuring unit. One drawback of this approach was signal dispersion in the cable. Generally, dispersion is a process by which an electromagnetic signal propagating in a physical medium becomes degraded due to various wave components, or frequencies, of the signal propagating at different velocities within the medium. Dispersion reduces the edge-rate or rise/fall times of the signals thereby degrading the ability of the system to accurately detect arrival time, and hence, position of the object.
In yet another class of prior systems (cf. U.S. Pat. Nos. 3,419,865; 3,680,121; and 4,916,455), measurement schemes were implemented at each of the monitoring stations to produce a digital result indicative of arrival times, angle of arrival, or other value. Advantageously, these systems conveyed digital data via interconnecting cables; and hence, position accuracy was not affected by cable dispersion. However, a drawback of this approach relates to the fact that these systems are plesiochronous, or “nearly” synchronous; i.e., timing reference signals were arbitrarily close in frequency (within some specified limits), but were not sourced from the same clock signal. Thus, over some period of time, the timing reference signals drifted with respect to each other. As each monitoring station had an independent clock source, small differences in clock frequencies degraded accuracy in position measurement.
Yet another class of prior systems included synchronous systems, i.e., those in which the timing reference signals were derived from a common source. In some synchronous systems (cf. U.S. Pat. Nos. 5,317,323 and 6,094,169), a local timing reference clock was derived from a Global Positioning Satellite (GPS) timing source. While this was suitable for frequency synchronization in benign outdoor conditions, monitoring stations operating indoors or in urban environments could not generally rely upon receiving a GPS timing signal, and consequently, object location could not be determined.
U.S. Pat. No. 5,119,104, for example, describes synchronization scheme in which a timing reference clock was provided at each monitoring station receiver by way of a local area network (LAN) cable. At each monitoring station, the clock signal incremented a digital counter that latched a count value upon receiving an RF signal arriving at an associated receiver of the monitoring station. Advantageously, this particular approach guaranteed that all counters operate at the same clock frequency. However, a drawback was the lack of a provision to reset the counters or otherwise control the relative phase between them. Non-compensated phase offset between counters degraded position accuracy. Furthermore, in the system described in the '104 patent, each of the monitoring stations included a data communication controller that responded to the receipt of an object tag transmission and, upon receipt thereof, sent a corresponding time of arrival (TOA) detection packet to a centrally located processor. In other words, such system was interrupt-driven where receipt of a tag transmission signal invoked an interrupt. A serious drawback of this approach was that, upon receiving a first tag transmission, the system was temporarily “disarmed” and thus unable to process a second tag transmission until the network completed the transfer of measurement data. Thus, it was possible that one or more tag transmissions were lost in the process.
Phase offset between counters can be controlled by a synchronizing or counter reset signal. U.S. Pat. Nos. 3,680,121 and 4,916,455, for example, disclose location systems utilizing an RF synchronizing signal that was transmitted to each monitoring station in the monitored region. To avoid interference, the synchronizing signal was transmitted at a frequency distinct from that of the tag transmission. Thus, one drawback of this approach was that each monitoring station had to be equipped with two distinct RF receivers—a first to sense the tag transmission and a second to sense the synchronization signal. Alternatively, the system disclosed by U.S. Pat. No. 3,419,865 included a cable interconnecting a central unit and each monitoring station to enable “adjusting their time clocks to precise mutual synchronization.” A drawback of this approach, however, was signal dispersion in the cable, which reduced pulse sharpness and timing accuracy of the synchronizing signal.
Synchronizing or calibration methods applicable to radio frequency location systems are also known (cf. U.S. Pat. Nos. 4,916,455; 5,119,104; and 6,094,169). A general synopsis of a calibration technique is provided in the '455 patent, in which it is stated that “[i]n order to achieve the high accuracy, the system was periodically calibrated. System calibration was accomplished by periodically transmitting a modulated signal (with a unique calibration identity code) from a known location. The transition times of arrival derived therefrom were then transmitted to a central analyzer for time-difference processing. The resulting time differences were then compared to known values and error magnitudes were then used to compensate corresponding station-pair time differences resulting from other unknown-location transmissions.”
The need for calibration is also summarized in the '104 patent as follows. “To operate the radiolocation system with TOA resolution in nanoseconds, minute changes in circuit operational parameters and propagation characteristics, such as might result from changes in temperature and humidity within the facility, had to be taken into account. Such changes were accommodated through system calibration”.
Another problem unique to object location or asset tracking is that, in order to accurately determine position, a minimum number of receivers at the monitoring stations (i.e. typically three receivers) must have a direct (i.e., a line-of-sight or, at most, an attenuated line-of-sight) transmission path. However, due to the nature of indoor environments, there may only be a limited number of such direct transmission paths. For example, walls, machinery, containers, and other materials may create signal attenuation or even complete signal blockage. Thus, there may exist certain zones within the monitored area in which position accuracy may be degraded for lack of adequate signal reception. A solution to this problem was to provide redundant monitoring stations. However, in providing such redundancy, it becomes possible, and in fact likely, that more than the minimum number of monitoring stations will receive a given transmission. Such a system is often referred to as an “over-specified” or “over-determined” system.
A potential drawback of using an over-determined system relates to the fact that hyperbolic ranging algorithms can calculate more than one mathematically valid position. That is, ambiguities in position determination can arise. Various techniques have been applied to address this issue. For example, U.S. Pat. No. 5,166,694 discloses a method of computing a vehicle location in an overdetermined system. One aspect of the '694 patent is the use of a pre-filter to “remove any signals that were corrupted by anomalies in the propagation of the transmitted signal.” In particular, the specification thereof describes a “multipath feasibility circle” that is determined by a system parameter that is an estimated maximum speed of the vehicle containing the transmitter. A drawback of this approach is that it is possible for a signal to have a propagation anomaly and yet not produce an error sufficiently large enough to cause it to be filtered out.
In commonly-owned U.S. Pat. No. 6,054,950 referenced above, ultra wideband (UWB) waveforms were employed to achieve extremely fine, centimeter-type resolution because of their extremely short (i.e., subnanosecond to nanosecond) durations. This patent also treats the use of UWB, or short pulse, technologies to provide an improved radio locating system and method for asset tracking that addresses the identified shortcomings of prior systems. However, the methods identified in the '950 patent are equally applicable to wideband pulse and spread spectrum RF technologies.